One type of transistor known in the art is a Schottky-barrier metal oxide semiconductor field effect transistor (Schottky-barrier MOSFET or SBMOS). As shown in FIG. 1, the SB-MOS device 100 comprises a semiconductor substrate 110 in which a source electrode 120 and a drain electrode 125 electrode are formed, separated by a channel region 140 having channel dopants. The channel region 140 is the current-carrying region of the substrate 110. For purposes of the present invention, the channel region 140 in the semiconductor substrate extends vertically below the gate insulator 150 to a boundary approximately aligned with the bottom edge of the source 120 and bottom edge of the drain 125 electrodes. The channel dopant concentration profile typically has a maximum concentration 115, which is below the source 120 and drain 125 electrodes, and thus outside of the channel region 140. For the purpose of the present invention, channel dopants are not constrained to be provided exclusively within the channel region 140, but may be found in regions substantially outside of the channel region 140.
For a SB-MOS device at least one of the source 120 or the drain 125 contacts is composed partially or fully of a metal silicide. Because at least one of the source 120 or the drain 125 contacts is composed in part of a metal, they form Schottky or Schottky-like contacts 130,135 with the substrate 110 and the channel region 140. A Schottky contact is defined as a contact formed by the intimate contact between a metal and a semiconductor, and a Schottky-like contact is defined as a contact formed by the close proximity of a semiconductor and a metal. The Schottky contacts or Schottky-like contacts or junctions 130, 135 may be provided by forming the source 120 or the drain 125 from a metal silicide. The channel length is defined as the distance from the source 120 contact to the drain 125 contact, laterally across the channel region 140.
The Schottky or Schottky-like contacts 130, 135 are located in an area adjacent to the channel region 140 formed between the source 120 and drain 125. An insulating layer 150 is located on top of the channel region 140. The insulating layer 150 is composed of a material such as silicon dioxide. The channel region 140 extends vertically from the insulating layer 150 to the bottom of the source 120 and drain 125 electrodes. A gate electrode 160 is positioned on top of the insulating layer 150, and a thin insulating layer 170 surrounds the gate electrode 160. The thin insulating layer 170 is also known as the spacer. The gate electrode 160 may be doped poly silicon. The source 120 and drain 125 electrodes may extend laterally below the spacer 170 and gate electrode 160. A field oxide 190 electrically isolates devices from one another. An exemplary Schottky-barrier device is disclosed in Spinnaker's U.S. Pat. No. 6,303,479.
Another type of MOSFET transistor known in the art is a conventional impurity-doped source-drain transistor or conventional MOSFET. This device is similar to the SB-MOS device shown in FIG. 1. The key difference is that the metal source-drain regions 120,125 of the SB-MOS are replaced with impurity doping in the semiconductor substrate for the conventional MOSFET.
One of the important performance characteristics for a MOSFET device is the drive current (Id), which is the electrical current from source to drain when the applied source voltage (Vs) is grounded, and the gate (Vg) and drain (Vd) are biased at the supply voltage (Vdd). Drive current is one of the important parameters that determines circuit performance. For example, the switching speed of a transistor scales as Id, so that higher drive current devices switch faster, thereby providing higher performance integrated circuits.
FIG. 2 shows the relationship of drive current (Id) 232 for varying gate voltage (Vg) and drain voltage (Vd) 231 for a SB-MOS and a conventional MOSFET. One characteristic of SB-MOS device Id-Vd curves is the sub-linear shape for low Vd 231, as shown by the solid lines 210,215,220,225,230. Each of the Id-Vd curves 210,215,220,225,230 has a different Vg. The Id-Vd profile at low Vd is known as the turn-on characteristic. Conventional MOSFET transistor technologies have a linear Id-Vd turn-on characteristic at low Vd, as shown by the dashed lines 235,240,245,250,255 in FIG. 2. Each of the Id-Vd curves 235,240,245,250,255 has a different Vg. The sub-linear Id-Vd turn-on characteristic of the SB-MOS device increases as the channel length decreases and can potentially reduce transistor performance, possibly reducing the effective switching speed of the device for example. Sub-linear turn-on has been observed in the literature and referenced as a reason why SB-MOS devices will not be of practicable use in integrated circuits (B. Winstead et al., IEEE Transactions on Electron Devices, 2000, pp. 1241-1246). Industry literature consistently teaches that the Schottky barrier height φb should be reduced or made less than zero in order to minimize the sub-linear turn-on phenomenon and thus to make SB-MOS device performance competitive with alternative MOSFET device technologies (J. Kedzierski et al., IEDM, 2000, pp. 57-60; E. Dubois et al., Solid State Electronics, 2002, pp. 997-1004; J. Guo et al., IEEE Transaction on Electron Devices, 2002, pp. 1897-1902; K. Ikeda et al., IEEE Electronic Device Letters, 2002, pp. 670-672; M. Tao et al., Applied Physics Letters, 2003, pp. 2593-2595).
There is a need in the industry for teaching a SB-MOS device and method of fabrication that provides a means for improving the turn-on characteristic thereby providing improved performance.